Rotax Piston Meltdown

[waterboy2110]

Originally Posted by aerofurb

There seemed to be some intimation earlier in the thread that the RV-12 uses a non-standard air filter set up (dual K&N filters taking air from inside the cowling).

In my experience, this is standard on non-certified 912 installations, with certified installations (Tecnam P2002JF and P2006T) using a single airbox with muffler fed hot air for carb heat.

Seeing as Rotax supply the filters, surely they don't consider it a problem?

It's mentioned in the service letter. Rotax-Owners also mentioned it. I couldn't tell you if the carbs were set up to run in hot or cold air but I would assume when tested at the factory they're sucking up cool air.

Carb heat robs the engine of power. Getting cool ram air might make a noticeable difference. Not sure if the Rotax air box would even fit under the cowl.

- SL-912-016R1
Excessively high intake air temperature.
Fresh air intake receiving hot air (e.g. carburetor heat on,
engine installation). Incorrect use of carburetor heat.

[rvbuilder2002]

As mentioned by Jerry, the use of individual filters on the carbs is very common with many different aircraft manufacturers/designs.

The filters are supplied by Rotax with the engine and in fact are custom designed for them by K&N.
Rotax owns the distribution rights. You can not buy that particular filter anywhere other than a Rotax parts dealer.

It would be incorrect to assume that induction air being drawn from inside a cowling would be of equivalent temp to what would be provided by a carb heat system. Would it be warmer than ambient outside air. Sure, but no where near as high as from carb heat supplied via the Rotax airbox.

[waterboy2110]

Originally Posted by rvbuilder2002

As mentioned by Jerry, the use of individual filters on the carbs is very common with many different aircraft manufacturers/designs.

The filters are supplied by Rotax with the engine and in fact are custom designed for them by K&N.
Rotax owns the distribution rights. You can not buy that particular filter anywhere other than a Rotax parts dealer.

It would be incorrect to assume that induction air being drawn from inside a cowling would be of equivalent temp to what would be provided by a carb heat system. Would it be warmer than ambient outside air. Sure, but no where near as high as from carb heat supplied via the Rotax airbox.

Perhaps, in this case the plane wasn't flying - just building heat. The fact that there is no carb heat on the 12 would indicate the temp of the air in the cowl is enough to avoid any carb ice. Was this tested? Do you have any data?

[AndrewR]

Originally Posted by aerofurb

Seeing as Rotax supply the filters, surely they don't consider it a problem?

Rotax supply the filters, but will tell you it's the airframe designer's problem to feed them cool air. But it is certainly very common to have the carbs taking hot air from inside the cowl.

My own aircraft is possibly the worst, as the carbs are sucking hot air from the radiator. I don't like it, but haven't figured out how to change it yet.

As with all these things none are the cause on their own, but they all reduce the available margin.

[AndrewR]

I had a look at your blog and the heads look like the post 2013 heads, so the measured temperature is coolant temperature and the limit is 120C. The actual CHT isn't measured.

Looking at the charts, it looks like the engine was pretty hot before takeoff - I'm not sure whether something unusual may have happened there? Maybe a long downwind taxi limiting airflow over the radiator? Whatever it was, it looks like the engine barely had enough cooling capacity during that low power period prior to takeoff. It looks like the coolant reached 105-110C on the ground.

I'm not sure of the dynamics of the cooling system when you then go to full power - my SWAG is that you have maybe 5 seconds where the cooling in the heads is very limited, before cooler water from prop blast through the radiator arrives.

One problem with liquid cooled engines is that when the metal is above the coolant boiling point, a layer of vapor can form and insulate the metal from the coolant. Without cooling, that spot then gets even hotter. That's why overheating can be so catastrophic for liquid cooled engines. The difference between adequate cooling and failure might be only a few degrees.

Prior to takeoff the coolant temperature was around 104C. I don't know the likely difference between head and coolant temperature, but head temperature is presumably somewhat higher. The coolant boiling temperature is probably around 130.

A possible theory:
On applying full power, before the cooler water arrived from the radiator, that cylinder just tipped over the point where the hottest areas boiled. This can be quite localized, with the vapor condensing back into the coolant but it still prevents that spot from cooling.
The hot spot in the head triggered some detonation, which further heated the head in a feedback loop until the damage was done.

The piston damage looks like detonation damage on the top (coolant outlet, presumably hotter) side, and debris damage on the bottom.

It makes me wonder whether the 120C temperature limit is OK for steady state i.e. in flight, but a lower limit might be needed before going to takeoff power...

[rv6ejguy]

Originally Posted by AndrewR

The piston damage looks like detonation damage on the top (coolant outlet, presumably hotter) side, and debris damage on the bottom.

IMO this failure is not due to detonation as I outlined in a previous post. There is a big difference in failure modes and length of time to failure between detonation and pre-ignition. They are not the same at all.

[aerofurb]

I recently had a chance to discuss the Rotax SL-912-016 with a UK-based Rotax engineer who does Rotax maintenance lectures and used to work at Rotax.

Apparently he was involved with the issue of SL-912-016 and said the majority of problems were on Katanas with CS props that were doing a lot of circuit flying and operated a lot with the prop pulled back.

It was his opinion that short term WOT wasn't an issue above 5000/5100 rpm. No doubt Rotax build in a safety factor into the SL.

Regarding the Rotax airbox used on the certified installations, to clarify: the airbox is fed with cold air (from a single filter) for normal use, with hot air on demand from inside the muffler shroud for carb heat.

Slightly off topic but may be of interest, we use this system:

http://www.skydrive.co.uk/proddetail.asp?prod=CH-912-3

which has the advantage of warming the carbs to prevent ice forming. As the induction air remains cold there is no loss of power.

[jnmeade]

Rotax coolant boiling discussion.
http://www.rotax-owner.com/pdf/ROAN%20cooling.pdf

[waterboy2110]

I've changed the X Axis on the charts to session time.

There was a fair bit of time holding short waiting for a regional to land on the crossing runway. The temps were still in spec per Rotax and the Vans 12 EFIS config but clearly not enough margin for safety.

I'll pull the heads this week and also get some clearer pics of the piston. It melted down near the coolant outlet / exhaust valve.

Also - there are a couple posts on age of engine and spark plugs etc.
The engine was delivered May 2016. It had 47 hours on it at time of failure and was using the original plugs (the automotive style NKG's).

[DanH]

Originally Posted by waterboy2110

I've changed the X Axis on the charts to session time.

Is session time in seconds?

Your RPM chart went missing.